1. Field of the Invention
The present invention relates in general to methods and apparatuses for polishing workpieces such as semiconductor wafers, and relates in particular to a polishing apparatus for processing a surface of a so-called xe2x80x9cdevice waferxe2x80x9d including various elements, wiring patterns, or patterned irregularities formed thereon, into a flat mirror surface.
2. Description of the Related Art
In recent years, the integration density of semiconductor devices has become progressively higher, which leads to finer interconnecting wirings and smaller integrated devices. In a manufacturing process for such semiconductor wafers, it is sometimes necessary to provide a step for removing films formed on a device wafer surface by polishing to planarize the surface.
Such process is necessary in the following cases:
(1) In a case of polishing a wafer having multi-layered circuits,
i) materials such as SiO2, SiOF, and CF for planarizing inter-layer films are polished,
ii) W, Al or Cu is polished after embedding plugs, or
iii) Al or Cu is polished after embedding wirings.
(2) In a case of manufacturing MOSFET (Metal Oxide Semiconductor Field Effect Transistor),
i) polycrystalline Si is polished after forming shallow trenches, or
ii) SiO2 is polished after forming various electrodes.
One of the available planarization methods is a chemical-mechanical polishing (CMP) process using an apparatus as shown in FIG. 22. The polishing apparatus comprises a turntable 9 having a polishing cloth (pad) 3, and a top ring assembly 10. The top ring assembly 10 comprises a top ring 13 for holding a semiconductor wafer 20, a top ring shaft 48 for providing the top ring 13 with rotational forces and pressing forces, and a ball 47 forming a universal joint for tiltably coupling the top ring 13 to the top ring shaft 48. The top ring 13 comprises an elastic mat 42 at a bottom surface to hold the wafer 20 through the mat 42. The top ring 13 has a cylindrical retainer ring 16 on its outer periphery to hold the wafer 20 so that the wafer 20 does not disengage from the bottom surface of the top ring 13 while polishing.
By such construction, the wafer surface is polished into a mirror surface while the wafer 20 is between the turntable 9 and the top ring 13; a certain pressure is applied by the top ring assembly 10 between the wafer 20 and polishing cloth 3; the turntable 9 and the top ring 13 are respectively rotated; and a polishing solution (slurry) Q including abrasive particles is supplied to the top surface of the polishing cloth 3.
One of the problems of the conventional chemical mechanical polishing (CMP) process is that, during the initial stage of polishing the device wafer having patterned irregularities, raised regions of the surface structure are preferentially removed, but depressed regions are also gradually removed. Therefore, irregularities of the surface are difficult to eliminate. This is because the combined use of the relatively soft cloth and the slurry solution containing abundant free abrasive particles applies chemical mechanical polishing effects not only to the raised regions but also to the depressed regions of the semiconductor surface structure. FIG. 23 illustrates such problems of the conventional CMP; showing irregularities caused by a raised region and depressed region of a surface film on the vertical axis, and relative time on the horizontal axis. This graph shows that, after a relative polishing time of 1, the raised regions are polished from a thickness of about 27,000 angstroms to 16,000 angstroms, and the depressed regions are also polished from 20,000 angstroms to 16,000 angstroms, and thus the irregularities are eliminated. FIG. 24A shows surface profiles in an initial stage, FIG. 24B in a middle stage, and FIG. 24C in a final stage of polishing. As illustrated in these drawings, irregularities are very difficult to be removed, that is, step height reduction rate is small, and consequently, such polishing is a time-consuming operation.
Another problem relates to xe2x80x9ca pattern dependencyxe2x80x9d of polishing due to a combination of the relatively soft polishing cloth and the slurry containing abundant abrasive particles. The pattern dependency is a difficulty in obtaining a completely flat surface, resulting from an already existing unevenness pattern on the unpolished surface. This is caused by the polish rate difference between the coexisting fine and coarse irregularities on the polished surface. The locations with fine irregularities are polished at a higher rate than the locations with coarse irregularities, thus resulting in a large-scale surface unevenness on the polished workpiece surface.
Also, an xe2x80x9cedge wearxe2x80x9d problem may occur, in which an outer periphery (edge) is more intensively polished than the inner region of the workpiece surface, because the polished workpiece is plunged into the elastic polishing cloth.
Another problem is that the polished workpiece is shaped like a dish due to selective polishing of the edge portions. This is caused by uneven distribution of the polishing solution supplied between the polishing cloth and workpiece from the outside to the central region, resulting in that the central region of the polished surface is supplied with a smaller amount of abrasive particles.
Furthermore, as described above, this method consumes a large quantity of suspension (slurry) including abundant expensive abrasive particles, so that environmental problems and a processing cost are also large.
Thus, a method has been developed, which uses a so-called fixed abrasive polishing tool (such as an abrading plate) which comprises cerium-oxide (CeO2) or other abrasive particles bound in a binder comprised of phenol resin, etc. This method uses an abrading plate harder than the conventional polishing cloth, which allows to preferentially polish the raised portions of the uneven surface, while leaving the depressed portions unpolished. Consequently, thus, absolute flatness of the polished surface can be obtained easily. Furthermore, the fixed abrasive polishing tool can provide a self-stopping function, depending on its composition, in which the polishing rate is remarkably lowered when the surface becomes flat after being rid of the irregularities. Thus, it can automatically stop further substantial polishing to avoid excessive polishing. Also, since it is not necessary to use a suspension (slurry) containing a large amount of abrasive particles, the environmental problems and processing cost can be lowered.
However, polishing using a fixed abrasive polishing tool has the following problems. That is, although a flat surface without scratches is necessary to produce a wafer suitable for producing devices thereon, the use of the hard tools causes the creation of many scratches on the polished surface of the workpiece while providing a highly flat surface.
Therefore, conventional fixed abrasive polishing tools for polishing the semiconductor wafers were allowed to have a limited choice of binder materials, and have been used in a narrowly balanced composition range of the abrasive particles, the binder and porosity. However, the device wafers to be polished comprise patterns of various materials such as: a silicon substrate, polycrystalline silicon films, oxide films, nitride films, and wiring layers comprised of aluminum or copper. Thus, it has been practically difficult to prepare various fixed abrasive polishing tools corresponding to the various polished subjects for preventing scratches while obtaining a stable removal rate with a small step height reduction rate.
It is an object of the present invention to provide a polishing method and apparatus capable of concurrently establishing a stable removal rate, a small step height reduction rate, and a reduction of defects on the polished surface for various kinds of polished subjects, while providing fewer environmental problems and requiring lower processing costs.
Such problems are solved by a method for polishing a surface of a semiconductor device wafer comprising: first polishing a surface of the semiconductor wafer by a first fixed abrasive polishing method; and finish polishing the polished surface of the semiconductor wafer by a second fixed abrasive polishing method different from the first fixed abrasive polishing method. The device wafers have been already explained in the description of the back ground of the invention. In a first polishing process using a first fixed abrasive polishing method, the wafer is processed to have a certain film thickness removed and a high flatness surface is realized. And in a finish process using a second fixed abrasive polishing method, a further thickness adjustment and/or scratch elimination is conducted while maintaining the high flatness, thereby realizing a high quality processing. It is not necessary to perform each of the first and finish processes by a single step process. One or both processes may be comprised of plural steps using one or plural apparatuses. Additional steps may be applied before and/or after those processes.
With the planarization method according to the present invention, it is basically not necessary to supply a slurry including free abrasive particles, and thus a polishing solution without abrasive particles is sufficient. Thus, it has the advantages of fewer environmental problems in waste liquid processing, and a lower cost load from consumable supplies such as slurries. Abrasive particles are directly supplied from the polishing surface of the fixed abrasive polishing tool, to thereby present a uniform abrasive particle distribution on the wafer surface to be polished, thus allowing the provisions of a uniform and high quality processed surface. If abrasive particles should stay in the slurry for a longer period, secondary coagulation may create larger particles to cause scratching of a polished surface. However, this secondary coagulation will not happen in the process of the present invention, and thus a stable polishing process is realized.
In the past, it was commonly acknowledged that fixed abrasive polishing is not suitable for use in finish polishing for removing remaining scratches. This is because fixed abrasive polishing tools are generally hard, and abrasive particles are also harder than those used in non-fixed abrasive polishing methods. Needless to say, different polishing tools, or different conditions such as pressing pressures or sliding speeds should be applied corresponding to the purposes of each of the first and finish processes. That is, the first polishing process aims at obtaining an overall flat surface by preferentially removing the raised portions of the irregularities, and the finish process aims at removing fine scratches remaining on the flattened surface. The inventors have established the present invention in a process of trying to adapt the fixed abrasive polishing method to finish polishing by decreasing the factor of mechanical polishing in the chemical mechanical polishing (CMP).
The second fixed abrasive polishing method may use a second fixed abrasive polishing tool different from a first fixed abrasive polishing tool used in the first fixed abrasive polishing method. As described above, the first polishing process aims at obtaining a highly flat surface, and the finish process is for removing scratches. Such different purposes are well established by using different polishing tools suitable for the respective processes.
The second fixed abrasive polishing tool may be softer than the first fixed abrasive polishing tool. Or the second fixed abrasive polishing tool may have a lower elastic modulus than the first fixed abrasive polishing tool. A high quality surface polishing can be achieved by using, after the first polishing process, a second polishing tool having a lower elastic modulus (average compressive elastic modulus) than the first polishing tool. Here, scratches are effectively removed by a synergistic effect of the lower elastic modulus tool with the above described processing characteristics of the fixed abrasive polishing tool. A suitable hardness for the first fixed abrasive polishing tool is Vicker""s Hardness of 10xcx9c70, and an elastic modulus of more than 500 kgf/cm2 (4900 N/cm2), preferably 500xcx9c50000 kgf/cm2 (4900xcx9c490000 N/cm2), and more preferably 1000xcx9c10000 kgf/cm2 (9800xcx9c98000 N/cm2). On the other hand, a suitable hardness for the second fixed abrasive polishing tool is, Shore""s Hardness of 5xcx9c60, preferably 15xcx9c40, or an elastic modulus of less than 1000 kgf/cm2 (9800 N/cm2), preferably less than 700 kgf/cm2 (6860 N/cm2), and more preferably 100xcx9c600 kgf/cm2 (980xcx9c5880 N/cm2). Materials having a cellular, hollow or foamed structure are especially suitable for the second fixed abrasive polishing tool, such as a binder comprised of a foamed resin.
The reason for overlapping of the suitable ranges of elastic modulus for the first and second polishing tools is that the suitable hardness differs according to the characteristics of the film to be polished, especially its hardness. A harder fixed abrasive polishing tool is used when polishing oxide films, and a softer fixed abrasive polishing tool is used when polishing copper, in general. Anyway, the second fixed abrasive polishing tool should be softer than the first fixed abrasive polishing tool, in summary.
The second fixed abrasive polishing tool may comprise second abrasive particles having a lower hardness than first abrasive particles in the first fixed abrasive polishing tool. It is known that the softer abrasive particles have less tendency of creating scratches than the harder abrasive particles in general. Therefore, a small amount of polishing using the fixed abrasive polishing tool having softer abrasive particles can remove scratches created by harder abrasive particles, thereby producing a high quality polished surface.
The second fixed abrasive polishing tool may have a more intensive abrasive particle self-generation ability than the first fixed abrasive polishing tool. A polishing tool having a greater ability can constantly have active abrasive particles exposed on its polishing surface, thereby enabling stable polishing. This is because, the number of abrasive particles contributable to the processing is large, the abrasive particles effective for processing are distributed uniformly on the polished surface, and the processing force load on each abrasive particle is minimized. Therefore, excessive abrasion by abrasive particles is prevented and a high grade polished surface with fewer scratches is produced. This effect of reducing the scratches is intensive when the particle size distribution is sharp, that is, has less variance. Furthermore, as the amount of self-generated abrasive particles is sufficient, it is not necessary to supply slurries from outside sources, thereby shortening the operation time.
Furthermore, in the conventional technologies, fixed abrasive polishing tools generally exhibit smaller self-generation effects as the abrasive particle size becomes finer, which prevented finer abrasive particles, advantageous for obtaining mirror polished surfaces, from being used. However, the method of the present invention has rendered it possible to obtain a mirror polished surface by using a fixed abrasive polishing method.
The second fixed abrasive polishing tool may comprise a higher porosity than the first fixed abrasive polishing tool. Fixed abrasive polishing tools are generally comprised of: abrasive particles for providing abrasive action; a polishing assistant effective or necessary for polishing; a binder for fixing those materials, and pores. By increasing the porosity among those compositions, abrasive particles are loosely bonded and easily self-generated. Thus, such a second fixed abrasive polishing tool comprising a higher porosity than the first fixed abrasive polishing tool has a higher self-generation effect, enabling the formation of a mirror polished surface.
The second fixed abrasive polishing tool may have a lower binder ratio than the first fixed abrasive polishing tool. By decreasing the binder ratio among the compositions, abrasive particles are loosely bonded and easily self-generated. Thus, the second fixed abrasive polishing tool having a lower binder ratio than the first fixed abrasive polishing tool has a higher self-generation effect, enabling a mirror polished surface to be provided.
The second fixed abrasive polishing tool may comprise a water soluble binder. When polishing by using such a polishing tool, the soluble binder dissolves into the supplied solutions such as purified water, a chemical agent or a slurry. This decreases binding grade for the abrasive particles in the tool to increase the amount of self-generated abrasive particles. This fixed abrasive polishing tool allows polishing in which the freed abrasive particles dominantly work over the as-fixed abrasive particles, without necessity of supplying a slurry from the exterior. Thus, a scratch-less surface can be obtained while maintaining a high flatness produced by the first fixed abrasive polishing tool. This second polishing tool also provides an advantage in that voids are formed on the polishing surface of the tool at the locations where the water soluble binder has dissolved, which serve to trap or hold foreign matter that is potentially detrimental to polishing, thereby preventing the creation of scratches.
The second fixed abrasive polishing tool may comprise second abrasive particles having a smaller diameter than first abrasive particles in the first fixed abrasive polishing tool. It is known that fixed abrasive polishing tools having smaller diameter abrasive particles are effective to reduce scratches. The reason is that the larger diameter particles are stuck in the worked surface while polishing. When having the same composition, the finer the abrasive particles are, the more the number of particles per unit volume of polishing tool is. Thus, effective particles are uniformly distributed on the worked surface for averaging and reducing the working force load on one abrasive particle. Thus, excessive work on the abrasive particles is suppressed and a high quality polishing with fewer scratches can be obtained. Furthermore, smaller abrasive particles have a larger specific surface area so as to provide a large amount of surface activity. Thus, polishing action based on the chemical reaction between the abrasive particles and the wafer is enhanced to improve finished surface flatness. Here, the narrower the particle size distribution, the more scratch elimination.
The second fixed abrasive polishing tool may additionally comprise elastic micro-particles embedded in the binder. By including such elastic micro-particles in the tool, the working force load on the worked surface from the abrasive particles is reduced, thus producing a high quality polished surface without micro-sized scratches.
The second fixed abrasive polishing tool may comprise a laminated configuration including an upper hard tool layer and a lower elastic layer. This underlying elastic layer beneath the hard tool layer gives the fixed abrasive polishing tool some elasticity for conforming to the worked surface of the wafer. Thus, the whole surface of the fixed abrasive polishing tool may contact the wafer, so that a uniform working pressure allows a scratch-less polished surface to be obtained. Since the surface layer is hard, the wafer does not plunge into the tool to the extent to degrade the height reduction rate.
The second fixed abrasive polishing method is preferably performed in a second condition different from a first condition in the first fixed abrasive polishing method. The first condition is aimed at obtaining a high flatness, and the second is to obtain a scratch-less surface by applying a lower working pressure, a higher relative speed, and a higher and sufficient amount of solution or chemical agent supply and so on. Thus, the method can produce a highly flat and scratch-less polished surface.
For example, it is preferable to, from the start of or in the mid-stage of the finish polishing process, load a lower working pressure than the first polishing process, and/or raise the relative speed between the wafer and the polishing tool, and/or increase the supply of polishing solution. Reducing the working pressure or raising the relative speed induces the generation of hydrodynamic forces at the sliding interface to thereby reduce the direct contact area between the polishing tool and the wafer. This moderates the working force vertically acting on the worked surface from the abrasive particles so as to provide scratch-less polishing. A sufficient supply of polishing solution facilitates the diffusion of ineffective fragments of abrasive particles, binders, or generated shavings into the solution and prohibits their deposition. The fragments are easily discharged because of the abundance of the supply of polishing solution, thereby preventing the generation of scratches.
The second fixed abrasive polishing method may use a second polishing solution different from a first polishing solution used in the first fixed abrasive polishing method. It is preferable to finish polish the wafer with a second fixed abrasive polishing tool while supplying purified water, after obtaining a highly flat surface by using a first fixed abrasive polishing tool while supplying solutions other than water. Here, hydration caused by OH radicals included in the purified water is applied to the worked surface in addition to the chemical action performed by the abrasive particles. Thus, a scratch-less worked surface can be produced after slightly polishing the surface using the second polishing tool, while maintaining the high flatness obtained by the first polishing tool.
In the finish polishing process using chemical agents, not only hydration, but other various chemical reactions are additionally applicable. Especially by supplying an agent for raising the viscosity of a liquid film formed between the fixed abrasive polishing tool and the wafer, abrasive particles moderately act on the worked surface to thereby reduce scratches. By supplying a slurry including abrasive particles, the polishing rate can be raised to shorten the processing time. Examples for agents to be supplied when polishing silicon or polycrystalline silicon films are: organic amines as a process acceleration agent, inorganic salts having pH buffering effects as a process performance stabilizing agent, organic macro molecules such as surfactants or the like as a mirror surfacing agent, and IPA (isopropyl alcohol) as a cleaning promotion agent.
Dressing of the polishing tool may be performed concurrently with polishing in the first polishing process and/or the finish polishing process. Dressing or reconditioning effects are not provided for the fixed abrasive polishing tool when processing a wafer after establishing a highly flat surface thereon. This is because the wafer surface has no remaining irregularities working for dressing the polishing tool, and the abrasive particle self-generation ability of the tool is lowered. An in-situ dressing (dressing concurrently performed with polishing) can forcibly activate the polishing surface of the tool so as to expose the unused abrasive particles on the polishing surface as if it is vigorously self-generating the abrasive particles. Thus, the finish process can produce a scratch-less polished surface after slightly removing the surface to reach a target thickness, while stably polishing by maintaining a high flatness produced by the first fixed abrasive polishing tool.
Furthermore, by dressing the tool, abrasive particles are promptly released so that a large amount thereof act on the polished surface. Thus, a sufficient number of substantially effective abrasive particles are supplied per unit polished surface area so that the pressing force applied by one abrasive particle is lowered so as to moderately act on the worked surface. When the abrasive particles are fine, it is preferable to add a dispersing agent such as a surfactant for facilitating the abrasive particles dispersed into the polishing solution.
It is preferable to clean the device wafer between the first polishing process and the finish polishing process. Foreign matter generated in the first polishing process, such as fragments of abrasive particles, binders or additives, or used or reacted chemical agents, may remain on the polished surface, which may cause some damage to the finished surface. By cleaning the wafer before the finish process, such damage can be prevented to provide a high quality polishing.
The first polishing process and the finish polishing process may be performed by using the same fixed abrasive polishing tool in different polishing conditions. A hard type polishing tool comprising fine abrasive particles, which are necessary for the finish polishing, is chosen as the same fixed abrasive polishing tool. The conditions for the finish polishing in comparison to the first process includes: lower working pressures, higher relative speeds, higher and sufficient supply of solution or chemical agent, and so on, that can prevent scratching. By adopting such conditions, a highly flat surface without scratches can be produced.
The first polishing process and the finish polishing process may be performed by using the same fixed abrasive polishing tool and different polishing solutions. A hard type polishing tool comprising fine abrasive particles, which are necessary for the finish polishing, is chosen as the same fixed abrasive polishing tool. It is preferable to finish polish the wafer by supplying purified water, after obtaining a highly flat surface by using a first fixed abrasive polishing tool and solutions other than water. Here, hydration caused by OH radicals included in the purified water is applied to the worked surface in addition to the chemical action caused by the abrasive particles. Thus, a scratch-less worked surface can be produced, while maintaining the flat surface obtained by the first polishing tool, by performing slight polishing using the second polishing tool.
In the finish polishing process, various chemical reactions other than hydration are additionally applicable. Especially by supplying an agent for raising the viscosity of a liquid film formed between the fixed abrasive polishing tool and the wafer, abrasive particles moderately act on the worked surface for reducing scratches. When supplying a slurry including abrasive particles, the polishing rate can be raised to shorten the processing time. Examples for agents to be supplied when polishing silicon or polycrystalline silicon materials are: organic amines as a process acceleration agent, inorganic salts having pH buffering effects as a stabilizing agent, organic macro molecules such as a surfactant or the like as a mirror surfacing agent, and IPA (isopropyl alcohol) as a cleaning promotion agent.
The first polishing process and the finish polishing process may be performed by using the same fixed abrasive polishing tool, and dressing of the polishing tools may be performed concurrently with polishing. A hard type polishing tool comprising fine abrasive particles necessary for the finish polishing is chosen as the same fixed abrasive polishing tool. Dressing effects or reconditioning effects are not provided for the fixed abrasive polishing tool when processing a wafer after establishing a highly flat surface thereon. This is because the wafer surface has no remaining irregularities to work for dressing the polishing tool, thus lowering the abrasive particle self-generation ability of the tool. An in-situ dressing (dressing concurrently performed with polishing) can forcibly activate the polishing surface of the tool to expose the unused abrasive particles on the polishing surface as if it is vigorously self-generating the abrasive particles. Thus, the finish process can produce a scratch-less polished surface by slightly removing the surface to reach a target thickness, while maintaining a high flatness produced by the first fixed abrasive polishing tool and a stable polishing performance.
Furthermore, by dressing the tool, abrasive particles are promptly released so that a large amount thereof act on the polished surface. Thus, a sufficient number of substantially effective abrasive particles are supplied per unit area of the polished surface so that the pressing force applied by the abrasive particles is lowered to moderately act on the worked surface. When the abrasive particles are fine, it is preferable to add a dispersing agent such as a surfactant for dispersing the abrasive particles into the polishing solution.
When the first polishing process and the finish polishing process are performed by using the same fixed abrasive polishing tool, the device wafer may be cleaned between the first polishing process and the finish polishing process. A hard type polishing tool comprising fine abrasive particles, which are necessary for the finish polishing, is chosen as the same fixed abrasive polishing tool. Foreign matter generated in the first polishing process such as fragments of abrasive particles, binders, or additives, or used or reacted chemical agents, may remain on the polished surface of the wafer to cause some damage to the finished surface at the finish polishing process. By cleaning the wafer before the finish process, such damage can be prevented to thereby provide high quality polishing.
When the first polishing process and the finish polishing process may be performed by using the same fixed abrasive polishing tool, the polishing tool may be cleaned or dressed between the first polishing process and the finish polishing process. When using the same tool for the first and finish processes sequentially, polishing solution or polish shavings may remain on the polishing tool, which may cause some damage to the surface to be finished at the finish polishing process. By cleaning the polishing surface of the tool before the finish process to remove such fragments of abrasive particles, binders, or additives (chemical agent) or reacted substances, such damage can be prevented to thereby provide high quality polishing. The cleaning processes may comprise a physical cleaning method for forcibly cleaning the tool, such as cleaning the object in a liquid flow of dressing liquid or purified water, water jetting the object, applying an ultrasonic wave in a dressing liquid or purified water, and dressing with a tool such as a brush, a roller, or a diamond dressing tool. Other processes may include using chemical agents, or radiating energy beams such as ultraviolet rays.
Another aspect of the invention is a method for polishing a surface of a semiconductor device wafer comprising a first polishing process wherein polishing of the device wafer is performed mainly based on a mechanical polishing effect, and a finish polishing process wherein polishing of the device wafer is performed mainly based on a chemical polishing effect.
In a first process mainly based on a mechanical polishing effect, raised portions of the wafer are preferentially polished using a hard fixed abrasive polishing tool, for example, and a high flatness can be obtained on the wafer surface. By further finish polishing the wafer mainly based on a chemical polishing effect, scratches created in the first process are removed to establish a high quality polishing. The chemical polishing effect can be increased by, changing the chemical agent or its concentration, when using the same polishing tool for both first and finish processes, for example. Also, reactive particles may be increased by performing an in-situ (concurrent) dressing while polishing, or mechanical polishing effects may be suppressed by lowering the polishing pressure in the polishing.
Furthermore, when using different polishing tools for the first and finish processes, the following methods are available in addition to the aforementioned methods: adding some chemical agents in the fixed abrasive particles; or using second fixed abrasive polishing tools including abrasive particles of a smaller diameter than the first fixed abrasive polishing tool. Smaller diameter abrasive particles have a larger specific surface area so that they are constantly providing a higher surface activity. Thus, they can provide not only reduced mechanical polishing action but increased chemical polishing action, to thereby provide a high quality processing.
In another aspect of the present invention, an apparatus for polishing a surface of a semiconductor device wafer comprises: a first polishing unit comprising a first fixed abrasive polishing tool, and a finish polishing unit comprising a second fixed abrasive polishing tool different from the first fixed abrasive polishing tool. For example, the second fixed abrasive polishing tool is designed to have a smaller elastic modulus or to be softer than the first fixed abrasive polishing tool. Thus, scratches are effectively removed by a synergistic effect of the lower elastic modulus tool with the above described processing characteristics of the fixed abrasive polishing tool.
A wafer holding member of the finish polishing unit may comprise a wafer retaining ring to surround the wafer and to contact with a surface of the second fixed abrasive polishing tool when polishing. A fixed abrasive polishing tool of a high elastic modulus has less deformability due to its rigidity, and thus has the feature that it can polish the overall surface of the wafer, including the edge portion, flat. Therefore, the guide ring surrounding the outer periphery of the wafer is not required to have the function of pressing the polishing tool around the wafer. On the other hand, the fixed abrasive polishing tool used in the finish process preferably comprises a guide ring having a polishing surface planation function to thereby prevent the polish rate from increasing at the edge portion.
The first and/or second fixed abrasive polishing tool may comprise a turntable, a translational motion table or a cup-type fixed abrasive polishing tool. When using a fixed abrasive polishing tool, only the raised portions of the undulated device wafer surface are preferentially processed. Thus, it is not necessary to provide a constant relative speed between the polishing tool and the worked surface of the wafer, and various types of driving mechanisms or polishing tools can be adopted. One example of an application of the translational motion table is a second fixed abrasive polishing tool utilizing a scroll type table movement, which leads to compactness of the overall facility and advantages in apparatus cost for the clean room, or ease of maintenance.
Another aspect of the invention is a method for polishing a surface of a semiconductor device wafer comprising: first polishing a surface of the semiconductor wafer with a first polishing tool; and finish polishing the polished surface of the semiconductor wafer for removing scratches remaining on the surface of the device wafer with a fixed abrasive polishing tool. In the past, it has been commonly understood that using a fixed abrasive polishing tool for the finish process for removing the scratches is disadvantageous. The reason for this is considered to be that the fixed abrasive polishing tools are generally hard and include larger abrasive particles compared to the non-fixed abrasive a polishing tools.
According to the present invention, the main or first polishing process may be a CMP process using conventional pads, and a high quality polishing without scratching is possible by finish polishing using a finish fixed abrasive polishing tool. Another example is comprised of a first CMP process using a conventional pad, a second CMP process using another conventional pad, and a finish process using a finish fixed abrasive polishing tool. Another example is comprised of a first CMP process using a conventional pad, a second CMP process using a hard fixed abrasive polishing tool, and a finish process using a finish fixed abrasive polishing tool. By performing polishing using free abrasive particles prior to a fixed abrasive polishing process, an overall general planarization can be established. By further performing the fixed abrasive polishing process for planarization and/or finishing, it is not necessary to raise the polishing rate or polishing pressure so that a scratch-less fixed abrasive polishing can be realized. By finish polishing with a fixed abrasive polishing process, finish polishing is performed without degrading the flatness of the surface obtained in the previous process.
A further example may be comprised of a first CMP process using a fixed abrasive polishing tool, a second polishing process using a conventional pad, and a finish polishing process using a finish fixed abrasive polishing tool. Here, a first step aims at obtaining a high flatness, and the second step is to process the generated xe2x80x9csharpxe2x80x9d scratches having edges of a small curvature into easy-to-remove ones having edges of a larger curvature. Thus, the final fixed abrasive polishing process can perform planarization as well as scratch elimination to thereby provide high quality processing.
The inventors have noticed that the fixed abrasive polishing process generally has a larger factor of mechanical polishing in a chemical mechanical polishing (CMP) process than the non-fixed abrasive polishing process. Thus, they made an effort to expand the chemical polishing factor in the CMP process in order to apply the fixed abrasive polishing to the finish polishing process, and have established the present invention. The inventors found that one of the significant parameters is a hardness of the polishing tool, and when the tool is soft, it can prevent or suppress the scratch creation. Suitable hardness for the finish fixed abrasive polishing tool is a Shore""s Hardness of 5xcx9c60, preferably 15xcx9c40, or an elastic modulus of less than 1000 kgf /cm2 (9800 N/cm2), preferably less than 700 kgf/cm2 (6860 N/cm2), and more preferably 100xcx9c600 kgf/cm2 (980xcx9c5880 N/cm2). Materials having a foamed or cellular structure are especially suitable, and thus a binder comprised of a foamed resin is preferable.